Touch sensitive devices allow a user to conveniently interface with electronic systems and displays by reducing or eliminating the need for mechanical buttons, keypads, keyboards, and pointing devices. For example, a user can carry out a complicated sequence of instructions by simply touching an on-display touch screen at a location identified by an icon.
There are several types of technologies for implementing a touch sensitive device including, for example, resistive, infrared, capacitive, surface acoustic wave, electromagnetic, near field imaging, etc. Capacitive touch sensing devices have been found to work well in a number of applications. In many touch sensitive devices, the input is sensed when a conductive object in the sensor is capacitively coupled to a conductive touch implement such as a user's finger. Generally, whenever two electrically conductive members come into proximity with one another without actually touching, a capacitance is formed between them. In the case of a capacitive touch sensitive device, as an object such as a finger approaches the touch sensing surface, a tiny capacitance forms between the object and the sensing points in close proximity to the object. By detecting changes in capacitance at each of the sensing points and noting the position of the sensing points, the sensing circuit can recognize multiple objects and determine the characteristics of the object as it is moved across the touch surface.
There are two known techniques used to capacitively measure touch. The first is to measure capacitance-to-ground, whereby a signal is applied to an electrode. A touch in proximity to the electrode causes signal current to flow from the electrode, through an object such as a finger, to electrical ground.
The second technique used to capacitively measure touch is through mutual capacitance. Mutual capacitance touch screens apply a signal to a driven electrode, which is capacitively coupled to a receiver electrode by an electric field. Signal coupling between the two electrodes is reduced by an object in proximity, which reduces the capacitive coupling.
Capacitive touch sensing devices often include two arrays of long, narrow electrodes in the form of a matrix. The arrays can be on two parallel planes and separated by an inter-electrode dielectric. Electrical parameters influenced by sensor construction, such as electrode resistance, inter-electrode (mutual) capacitance, and electrode capacitance to ground must be balanced with performance considerations. For example, high levels of parasitic mutual capacitance among electrodes may interfere with the measurement of small changes to mutual capacitance that occur due to a touch. While a reduction in parasitic mutual capacitance may be achieved by increasing inter-electrode dielectric thickness, this increases the thickness and weight of the touch sensor, and also decreases the capacitance-changing effect of a touch.
There are numerous other performance and construction considerations present when designing a touch sensor. For example, it can be desirable to shield touch signals from electromagnetic interference emitted from nearby electrical components. Capacitive coupling between a touching implement or finger and the lower electrodes can be equalized relative to the top electrodes. There also exists a desire for greater flexibility in the design of electrically conductive elements and a method for an improved manufacturing process for touch systems with customized sensors and unique electrode configurations.